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BTEC waves unit 1
|What happens when a ray of light enters a block of glass?
|the light will slow down and the ray will move towards the normal (slo-to)
|What is Snell’s law?
|Define the critical angle
|the angle to the normal for which a light ray, going from a more dense to a less dense medium, has a refracted ray at 90 to the normal
|The critical angle for a ray going from water to air is 49. What will happen to a ray arriving at an angle of 60?
|it will be totally internally reflected and stay inside the water
|The critical angle for a ray going from water to air is 49. What will happen to a ray arriving at an angle of 30?
|the refracted ray will come out of the water and will move away from the normal
|A ray arrives at a boundary with an angle of incidence of 20. The refractive index of the first medium is 1.40 and of the second medium 1.60. What is the angle of refraction?
|n1xsin(i)=n2xsin(r); 1.40xsin(20)=1.60xsin(r); sin(r)=1.40xsin(20)/1.60;r=17.4
|A ray goes from air to glass. The refracted ray inside the glass has an angle of refraction of 40. Would the angle of incidence be more, less or the same than the angle of refraction?
|As the ray from air to glass (a denser medium), the light slowed down, so the refracted ray moved closer to the normal. Ie the incident ray was further from the normal, so the angle of incidence was more than 40.
|A ray enters a block of glass at an angle of 40. And later comes out. What can you tell about the ray leaving the block of glass, compared to the ray entering the block?
|The two rays are parallel
|The refractive index of medium 1 is 1.35 and of medium 2 is 1.65. The angle of refraction is 40. Work out the Angle of incidence
|n1xsin(i)=n2xsin(r); 1.35xsin(40)=1.65xsin(r); sin(r)= 1.35xsin(40)/1.65; r=60.2
|Total internal reflection will not occur when a ray goes from air to glass. Explain why
|Because Total internal reflection occurs only when the ray goes from a denser medium to a less dense medium
|A ray of light goes from a liquid into glass. The angle of incidence is 29.2 and the angle of refraction is 26.6. The refractive of glass is 1.45. Work out the refractive index of the liquid.
|n1xsin(i)=n2xsin(r); n1xsin(29.2)=1.45xsin(26.6); n1=1.33
|State one advantage of cladding for optical fibres
|to protect the core
|CHALLENGE: what would happen if a glass core in an optical fibre was surrounded by a cladding with a reflective index bigger than the glass core?
|the rays of light would not be totalled internally reflected and the ray would leave the core
|A sound wave produced by a car backfiring 515m away is heard 1.50s later. What is the speed of sound in air?
|A sound wave has a frequency of 436 Hz. What is its period?
|A sound wave, going at a speed of 343m/s, has a frequency of 436 Hz. What is its wavelength?
|speed=wavelengthxfrequency; 343=wavelengthx436; wavelength= 0.787m
|A hiker slips and falls into a deep valley screaming all the way down, the opposite side of the valley is 685m away. His partner hears the echo of the scream 4.00s later. What is the speed of sound in air?
|It is an ECHO, so the sound wave bounced back, so the distance should be doubled 685x2=1370; speed=distance/time=1370/4.00=343m/s
|The wavelength of the sound travelling at 343m/s is 0.750m. What is the frequency?
|speed=wavelengthxfrequency; 343=0.750xfrequency; frequency=457Hz
|What is the period of the wave of the sound travelling at 343m/s with a wavelength of 0.750m.?
|TWO steps: Period=1/frequency; and frequency=speed/wavelength; so frequency=343/0.750=457Hz and therefore Period =2.19x10-3s
|A radio wave, a form of electromagnetic wave, has a frequency of 99.5MHz. What is its wavelength? (speed of light=3x10^8 m.s-1)
|CONVERT 99.5MHz=99.5x10^6Hz; frequency=speed/wavelength; wavelength= 99.5x10^6/3x10^8=0.331m
|A ripple tank produces waves with a frequency of 5.2 Hz and a wavelength of 14.8cm. What is the speed of its waves?
|CONVERT 14.8cm=0.148m; speed=wavelengthxfrequency; speed=5.2x0.148=0.770m/s
|Certain light waves have a frequency of 3.95 x 10^5 Hz and a wavelength of 759m. What is the speed of light?
|speed=wavelengthxfrequency;speed=3.95x10^5x759=3.00x108m/s (3 significant figures)
|A source emits light, in pulses, at a wavelength of 600nm. What is the frequency? (speed of light=3x10^8 m.s-1)
|CONVERT!! 600nm=600x10^-9m; speed=wavelengthxfrequency; 3x10^8=600x10^-9 x frequency; frequency=5x10^16Hz (did you remember the units?)
|A loud speaker produces sound waves in air of wavelength 0.68m and speed of 340 m.s-1. What is the frequency?
|speed=wavelengthxfrequency; 340=0.68 x frequency; frequency=500Hz (did you remember the units?)
|What are longitudinal waves?
|Sound waves and waves in a spring are longitudinal waves. In longitudinal waves, the vibrations are along the same direction as the direction of travel
|What are transverse waves?
|Light and other types of electromagnetic radiation are transverse waves. Water waves and S waves (a type of seismic wave) are also transverse waves. In transverse waves, the vibrations are at right angles to the direction of travel.
|How is a stationary wave produced?
|when two waves are travelling in opposite directions and superpose
|What is a node in a stationary wave?
|the point where the two superposed waves are in antiphase and where displacement is zero (ie on x-axis)
|What is an antinode in a stationary wave?
|the point where the two superposed waves are in phase and where displacement (amplitude) is maximum
|In a stationary wave diagram, where are the nodes and antinodes?
|nodes are the points on the x-axis where the two “waves” crossover; anti nodes are the “top of the wave”
|Using the stationary wave diagram, how would you work out the wavelength?
|The wavelength is the distance between TWO loops.
|How many loops would you draw for a FUNDAMENTAL stationary wave?
|How many loops would you draw for the second mode stationary wave?
|Explain why stationary waves are seen only at certain frequencies
|a string has a series of natural frequencies; corresponding to a number of half Wavelengths; a stationary wave is produced only when the frequency of the vibration generator produces waves of those wavelengths
|State one use of fibre optics in medicine
|endoscopy; key hole surgery
|Explain why there is total internal reflection in an optical fibre
|fibre optic cable denser (HIGHER REFRACTIVE INDEX) than outside/air/cladding; so when the incidence angle is greater than the critical angle, the ray is totally reflected
|What happens to the intensity of a signal (ie how strong is the signal) as the distance between emitter and receptor is DOUBLED?
|the intensity will decrease by a factor of TWO (the doubling) SQUARED (this is the inverse square law)
|Which part of the electromagnetic spectrum do these use: mobile phones, wifi, Bluetooth?
|Which of these emit the furthest and why? mobile phones, wifi, Bluetooth?
|mobile phones because the transmitters are more powerful
|What is the advantage of using microwaves rather than radiowaves?
|large amount of data can be transmitted (large bandwidth). Small transmitters/repeaters. Goes through the atmosphere so signal can be passed on to satellites..
|What is the advantage of using radiowaves rather than microwaves?
|Radio waves are diffracted around hills, so are able to reach most areas & are suitable for broadcasting. (no need for line of sight). They are reflected from the ionosphere so can be used for long distance communication. They are not affected by weather.
|State one disadvantage of radiowaves when used for communications
|When radio stations use similar transmission frequencies the waves sometimes interfere with each other.
|State one disadvantage of microwaves when used for communication
|microwaves are affected by the weather; are absorbed by water
|Explain what happens to the light when an optical fibre is bent dramatically
|No (or little) light would come out because the light in the cable hits the side of the cable at an angle less than the critical angle so light is not reflected
|State the advantage of using digital (rather than analogue ) signals in long distance communication
|Digital signals are clear and of high quality. They can carry a lot of data. (signal) can be regenerated. no loss of signal (no attenuation )
|All signals become weaker as they travel long distances, and they may also pick up random extra signals. This is called noise, and it is heard as crackles and hiss on radio programmes. Explain why noise does not affect digital signals as much
|Noise adds random information to signals. When signal is amplified, so is the noise. But noise is usually lower in amplitude than the amplitude of the digital. So amplifiers can ignore the noise and it does not get passed along.
|When light is passed through a slit, it produces a pattern on the screen. Explain why
|Light is diffracted.
|What are coherent waves?
|waves with a constant phase difference. (Note: They don't have to be in phase for this to be true.) They will have the same frequency (and wavelength)
|Explain how a laser beam passing through 2 slits will produce a series of dark and light bands on a screen
|Each slit acts as coherent source of light. If the waves are antiphase then destructive interference occurs, resulting in a dark patch on the screen. If the waves are in phase then constructive interference, resulting in a light patch on the screen.
|A violin string is 35 cm long and has a mass of 2.25 g. It produces a note of frequency 256 Hz when sounding its first harmonic. Find the tension in the string 
|T=mu x v-squared; mu=mass/length=2.25/35; v=frequencyxwavelength=256Hzx0.35m; T=51.6N